Start by put the dampening material on the large un-braced peices of sheet metal in your car, like the truck lid, then the metal inside the doors, then the rear deck below the back window (unless you have a hatchback). The large pieces of sheet metal in your car are the first to start vibrating, then check where the braces meet the sheet metal. A poor weld can make two peices of metal vibrate against each other. Also check all your screws and bolts and make sure that their tight, and any screws that hold your interior plastic paneling down, like door panels, etc.
By eliminating some vibration, theoretically, you should notice an increase in SPL, because vibration is a form of energy. And the metal of your car is absorbing and wasting that energy through vibration. When you eliminate that waste of energy you increase the SPL inside the car (if SPL is what you are looking for). But dampening will also help your sound quality, especially if you put sound dampening around where your speakers mount in your doors, dash, and rear deck.
